The IEEE 802.11s standardization committee group is currently working on an extension of the 802.11 standard for meshes. The current IEEE 802.11s standard specification, version D1.05, incorporated herein by reference, defines an IEEE 802.11 WLAN using the IEEE 802.11 MAC/PHY layers that supports both individually addressed and group addressed delivery over self-configuring multi-hop topologies. Mesh networks according to the 802.11s standard, or so-called meshes, operate as wireless co-operative communication infrastructures between numerous individual wireless transceivers. A mesh may be centralized or decentralized. Stations or mesh nodes (MP) in the mesh communicate with their neighboring adjacent nodes only and thus act as repeaters to transmit message data from nearby nodes to peers that are too far to reach. Terminology specific to the 802.11s standard will be used in the following paragraphs to illustrate the invention.
By definition, a mesh point MP is an IEEE 802.11 entity that contains an IEEE 802.11-conformant medium access control and physical layer interface to the wireless medium that supports mesh services as defined in the 802.11s standard. Timing of communication over a mesh is of paramount importance to permit efficient communication and prevent conflicts over the medium. The IEEE 802.11 standard and the IEEE 802.11s D1.05 specification propose timing rules that govern communications over the medium and, in particular, mechanisms on how to spread the knowledge of timings from one MP to its neighboring MPs. Because MPs can only communicate with neighboring MPs, information need be spread to at least second level MPs so that most conflicts can be avoided. In meshes, MPs are often engaged in periodic transmissions such as beacon transmissions and/or data communications in reserved time periods. Typically such transmissions are periodically announced in the time domain. These periodic announcements enhance the robustness and reduce competition of MPs for the medium.
Typically, in wireless communication systems, transmissions are vulnerable to interference as simultaneously transmitted frames may collide and corrupt each other. These systems are usually built in with a number of preventive measures to reduce the number of collisions. Examples from the IEEE 802.11 standard include the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and the Request to Send/Clear To Send (RTS/CTS) virtual carried sense protocols.
Medium access problems are exacerbated in systems where the node density is high and where hidden nodes exist. A prime example where such problems likely occur is in mesh networks. A solution to reduce collisions is to advertise MPs scheduled transmissions to other nearby MPs so that the information may be advertised further.
The IEEE 802.11s defines at least two mechanisms to spread timing information to second level peer MPs in the mesh.
The first mechanism concerns the transmission of beacon frames and is defined in section 7.2.3.1 Beacon frame format and section 7.3.2.65 Beacon Timing Element in the IEEE 802.11s D1.05 specification. Section 7.2.3.1 defines the format of the beacon frame that each MP periodically transmits to its neighboring MPs over the medium. A beacon frame includes a beacon timing element that will inform neighboring MPs of the timing of the current beacon frame and future beacon frames with respect to the specific MP. Section 7.3.2.65 gives a possible format for the beacon timing element and more details will follow hereinafter. The beacon timing element is used by an MP to advertise an offset between its timing synchronization function (TSF), or clock, and the mesh TSF. The beacon timing element may also be used to advertise the beacon timing information of the MP's neighbors. By advertising beacon timing information of its neighbors to all its neighbors, timing information may be spread to second level peer MPs. As a result, a given MP will know when to expect beacon transmission by a second level MP with whom it has got at least one common neighbor and will therefore not initiate transmissions with the common neighbor at the indicated time.
The second mechanism is a way to advertise the reservation of the medium by an MP for communication from or to the MP. Section 7.3.2.69 proposes one possible solution and to this respect, defines a so-called MDAOP advertisements element. It is used by an MP to advertise its mesh deterministic access state to its neighbors. The MDAOP advertisement element informs neighbors of when the MP is scheduled to either receive or transmit information data. In addition, the MDAOP advertisements element further includes similar information on MP's neighbors reported scheduled communication over the medium. Again, timing information is spread to at least the second level peers by each MP repeating the timing information on its neighbors as reported by the individual neighbors. Section 7.3.2.69 proposes a possible format.
Although the above mechanisms enhance communication robustness over the medium and prevent conflicts among MPs, a great amount of communication bandwidth is nevertheless dedicated to the spreading of the information. Beacon frames thus tend to be larger in size as the mesh density increases and may end up taking a great part of airtime.